Regex ? in lookbehind not consuming

Question

Lookbehind with ? is not consuming the :

Regex:

(?i)(?<=\bsubject:?).+$

Text:

Subject: asdf adsf

Match

: asdf adsf

I don't want the : to be included in the match

If I search on:
Subject asdf adsf
It matches:
asdf adsf
That is the behavior I want
It appears to treat the : as optional but not consuming

If I match on just subject:? it is greedy and includes the :

Answer 1

use this regex (?i)(?<=\bsubject:?)[^:].*$

Answer 2

You have a question mark after the : which will make the colon optional.

(?i)(?<=\bsubject:).+$

Answer 3

This works ok for me:

(?i)(?<=subject:).+$

Remove the extra question mark.

Answer 4

Unless I've misunderstood, I don't think you need a look-ahead for this?

If you're just after whatever follows Subject, then this is your regex:

Subject\:?(.+)$

And if you're after whatever follows Subject, subject or eg suBject then this is your regex:

(?i)Subject\:?(.+)$

And include your word-boundary \b again if that's still necessary:

\bSubject\:?(.+)$ or (?i)\bSubject\:?(.+)$

So, in C#.NET:

Regex r = new Regex(@"(?i)Subject\:?(.+)$");

Match m = r.Match("Subject asdf asdf");
Console.WriteLine(m.Groups[1]);

m = r.Match("Subject: asdf asdf");
Console.WriteLine(m.Groups[1]); 

//Both output ' asdf asdf' - you might want to trim this.  
//Or add optional space \s? after the optional colon \:? in your regex.

Or, rather than relying on magic constants to line up with implicitly numbered capture groups, explicitly name (and reference) the group:

string captureName = "yourcap";

Regex r = new Regex(@"(?i)Subject\:?(?<"+captureName+">.+)$");
Match m = r.Match("Subject asdf asdf");
Console.WriteLine(m.Groups[captureName]);